Surface preparation of titanium and titanium alloys for bonding materials thereto

ABSTRACT

A METHOD FOR PREPARING A TITANIUM OR TITANIUM ALLOY SUBSTRATE FOR ACCEPTANCE OF A COATING OF METAL OR OTHER MATERIAL THEREON WHEREBY THE SUBSTRATE IS TREATED WITH A BATH CONTAINING CHROMIC ACID AND SULFURIC ACID. COATINGS APPLIED AFTER PRETREATMENT ARE STRONGLY ADHERENT TO THE SUBSTRATE AND RESIST CRACKING, CHIPPING OR STRESS MIGRATION. A NICKEL STRIKE IS OPTIONAL AS FURTHER PRETREATMENT PRIOR TO METALLIC COATING. A CHROMIC ACID-SULFURIC ACID BATH IS USED IN SAID METHOD.

United States Patent SURFACE PREPARATION OF TITANIUM AND TITANIUM ALLOYS FOR BONDING MATE- RIALS THERETG Lewis Edwin Augustine, Santa Monica, Calif., assignor to Titanium Processors Incorporated N0 Drawing. Filed Feb. 5, 1970, Ser. No. 9,032 Int. Cl. C23f 7/26 US. Cl. 148-621 10 Claims ABSTRACT OF THE DISCLOSURE A method for preparing a titanium or titanium alloy substrate for acceptance of a coating of metal or other material thereon whereby the substrate is treated with a bath containing chromic acid and sulfuric acid. Coatings applied after pretreatment are strongly adherent to the substrate and resist cracking, chipping or stress migration. A nickel strike is optional as further pretreatment prior to metallic coating. A chromic acid-sulfuric acid bath is used in said method.

BACKGROUND OF THE INVENTION Field of invention This invention relates generally to a method for providing adherent coatings on titanium or titanium based alloys. More particularly, the invention relates to a technique for pretreating a titanium or titanium base alloy substrate prior to the formation of a coating thereon. According to the invention, the substrate surface is prepared for acceptance of the coating by treatment with an acid bath containing sulfuric acid and chromic acid. A nickel strike after treatment is optional for further preparation of the titanium surface for acceptance of a metallic coating.

Description of the prior art The use of titanium is now widely accepted for many industrial applications where a light-weight, high-stress resistant material is required. However, titanium having a very high affinity for oxygen is readily oxidized upon exposure to air even under mild conditions of heat and temperature. The oxidized surface is passive, possesses poor strength characteristics, is not strongly adherent to the titanium itself, and is diflicult to coat over.

It has been the practice in the art in using titanium to coat the titanium with other materials such as metals, paints, glues, epoxies and ceramic depending on the particular application of the finished product. Many problems have been attendant, however, in providing a galvanic barrier for titanium. On the one hand, metallic coatings cannot readily be adhered to titanium in its purest state, partly because of its afiinity for oxidation. Also as titanium has such a great affinity for oxygen, it is diflicuit when using conventional coating techniques to avoid formation of the oxide. Therefore, the coating is invariably applied to at least a partially oxidized surface. As the titanium oxide presents a passive surface, the resulting coatings are too readily chipped and peeled exposing the surface to further oxidation and degradation.

These difficulties have been found to be especially acute in the aircraft industry which is a major user of titanium for fasteners, rivets and other aircraft parts. Cadmium or nickel coated titanium is frequently used for highstress construction applications. Under use conditions, it has been frequently found that the coating will chip or peel from the titanium rivet or fastener, subjecting the parts to physical and galvanic deterioration. Even if special precautions are taken to insure that no oxide is formed prior to or during the coating operations, these techniques are costly and not practicable under many circumstances.

Particularly, it has been found that a certain degree of stress migration occurs whereby the metallic coating material migrates into the titanium substrate. This greatly reduces many of the more desirable characteristics of titanium.

While there have been many prior attempts to alleviate these problems, none have succeeded in providing both an adherent coating and in preventing the migration of coating material into the substrate. Ordinarily, the substrate surface is pretreated to physically or chemically roughen it and to remove the oxide surface. For example, parts are sand blasted, grit blasted or acid etched and coated before the oxide coating can re-form. Electroforming is also employed. While these techniques enable some degree of bonding of the coating to the substrate, the coatings are not very adherent and stress migration in the case of metallic coatings is not prevented. Moreover, as bonding actually occurs only at the etched portion of the substrate, the quality of the bonds are oftentimes poor and the coating remains subject to peeling and chipping.

SUMMARY OF THE INVENTION Accordingly, it is the primary object of this invention to provide a pretreatment technique to prepare a titanium or titanium base alloy substrate surface for the acceptance of a strong, adherent, non-degradable coating.

It is a further object of this invention to provide a strongly adherent coating on a titanium or titanium base alloy substrate which will effectively prevent the oxidative deterioration of the substrate metal and which will not tend to be degraded by stress migration.

It is still a further object of this invention to coat a titanium based substrate with a metallic coating which will not be degradable under high stress conditions and will not tend to migrate into the substrate.

It is a still further object to coat titanium objects such as rivets and fasteners with a sufficiently adherent coating so as to render them acceptable for use in the aircraft and construction industries.

These and other objects have now been provided herein by the technique of pretreating a titanium or titanium alloy substrate with a bath containing chromic acid and sulfuric acid prior to coating. While the exact nature of the effect of this pretreatment is not completely understood, it is believed that the pretreatment changes the surface of the titanium in part to a monoxide or sulfide either of titanium itself or of other metals or impurities present in the alloys. The surface composition is probably a mixture. While its composition is unknown, it is clear that after treatment of the substrate in a mixed acid bath, as is more fully described hereinafter, the substrate more readily accepts a continuous adherent coating. Materials such as meals, paints, glues, epoxies and ceramics have been applied after pretreatment and resist chipping, peeling, migration and other forms of degradation even under high stress and high temperature conditions.

In the case of applications whereby a metallic coating such as cadmium or chromium is desired to be plated on the titanium as for aircraft industry applications, a nickel strike may be used after the acid pretreatment described. This is accomplished by deposition of a thin layer of nickel from a nickel strike bath such as nickel sulfamate. The thin layer of nickel sets up a barrier to stress migration enabling desired metallic coatings to be readily plated on this surface.

After pretreatment and/or an optional nickel strike, the titanium surface may be coated by any conventional means such as electroless coating, electroplating, vapor deposition, dipping, brushing, etc. Among other materials that may be coated on titanium in this fashion, as will be more fully described, are zinc-based paints such as zinc chromate paints, rubber-based paints, and metallic and epoxy paints. Chromium trioxide is an especially desirable coating for certain applications as it forms a selflubricating coating. Because of the lack of friction between parts so coated, necessity for external lubrication is greatly reduced.

Where metallic coatings are to be applied, treatment with a nickel strike as indicated above is desirable prior to metal coating in order to set up a barrier against stress migration. Once the nickel strike has been applied, some common metallic coatings are cadmium for aircraft applications, chromium for lubricity, silver for conductivity, iron for its adhesive qualities and zinc as in paint bases. Coatings applied in accordance with the methods of this invention may be seen to be strongly adherent by means of drop testing, bend testing, scratch testing, etc.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The substrates to be treated in accordance with this invention in order to prepare them to accept a coating are titanium or titanium base alloys. They may be of any shape or size as for example sheets, wires, rods or finished articles such as fasteners or rivets. The terms titanium titanium base alloys are meant to include pure titanium and all alloys thereof in which titanium constitutes at least 50% of the metallic mix. Typical, but not limiting of such alloys are those having the following compositions:

Metals Percent Chromium 3.00 Iron 1.50 Oxygen 0.50 Nitrogen 0.04 Tungsten 0.08 max. Carbon 0.02 Titanium Balance Metals: Percent Chromium 2.70 Iron 1.30 Oxygen 0.25 Nitrogen 0.02 Carbon 0.02 Tungsten 0.04 Titanium Balance Typical also are the vanadium or aluminum-containing alloys such as those known as 6-4 titanium or 8-1-1 titanium. Also suitable for treatment is so-called commercially pure titanium which actually contains:

Metals Percent Iron 0.10

Nitrogen 0.02 Carbon 0.04 max. Tungsten 0.04 max. Oxygen Trace Titanium Balance The substrate surface is first cleaned by using conventional state-of-the-art techniques. For example, oils or organic soils are removed by soaking the work piece in a conventional alkaline type cleaner, such as one containing sodium hydroxide and suitable wetting agents. Diversey soak cleaner distributed by the Diversey Corporation and Wyandotte F-S are examples of two suitable cleaners. The substrate is then rinsed in deionized water and neutralized in a dilute sulfuric acid solution. Mechanical cleaning means may also be employed such as grit blasting, sand blasting, hydro blasting or vapor blasting.

The substrate surface is then roughened up to accept the coating. This may be done by physical means, but is preferably accomplished by subjecting the substrate to a mild etching bath containing hydrofluoric acid and nitric acid. As for example, one containing 10% by volume nitric acid and /2 by volume hydrofluoric acid. A typical etching bath would contain 1 /2 parts per gallon of nitric acid and 3 fluid ounces per gallon of hydrofluoric acid. After etching, the substrate is removed from the bath and thoroughly rinsed.

The pretreatment bath of this invention is essentially an aqueous mixture of sulfuric acid and chromic acid. A typical treating bath would have 33 ounces per gallon of chromic acid and 4 fluid ounces per gallon of sulfuric acid. The range of acceptable compositions of this treating bath, however, is from 2 to 50 ounces per gallon of chromic acid and from 1 quarter fluid ounce per gallon to 10 fluid ounces per gallon of sulfuric acid.

In a preferred, but optional embodiment, the treating bath will additionally contain from one quarter ounce to two ounces of silver either as the metal or the equivalent weight of a silver salt. One ounce per gallon is the optimum amount. It is not clear what effect the silver has on the coating process; however, it does protect the longevity of the bath. In addition, a trace amount of aluminum sulfate, although also optional, has been found desirable. Thus, in a preferred embodiment aluminum sulfate will be present in amount of approximately one half gram per gallon of bath.

A typical pretreatment bath according to this invention would be prepared by admixing 33 ounces per gallon of chromic acid, 4 fluid ounces per gallon of sulfuric acid, 1 ounce per gallon of silver as the metal and /2 gram per gallon of aluminum sulfate.

The pretreatment itself is preferably carried out in a lead lined vessel. Lead lined steel is typical. The parts are immersed in the bath on a titanium rack or in a titanium basket. Pretreatment is carried out at elevated temperature above approximately 215 Fahrenheit, preferably from 240 to 250 degrees Fahrenheit. The immersion time will vary with the temperature; however, it will typically be from 45 minutes to a maximum of 3 hours. A time range of 15 minutes to 3 hours depending on the particular alloy and part to be coated is used. When a titanium part of titanium alloy substrate is immersed in the treating bath, the surface gradually changes in color from a brown to a black. The time of treatment will be determined for most applications by the time required to produce this black color in the surface.

The acid bath can be applied to the titanium work piece by any method which permits contact of the bath with the substrate for the required time period and at the required temperature. For instance, the solution can be sprayed on to the substrate, or the substrate can be dipped or brushed for the required time period. Immersion is preferred for ease of handling hot acid solutions.

Following the chromic acid sulfuric acid pretreatment, any excess acid may be removed by rinsing the work piece with deionized water. The Work piece surface, once pretreated, is stable and may be set aside for an indefinite period of time. The pretreated substrate may now be coated by any conventional technique for forming coatings on metal substrates. Although, as indicated above, the exact surface composition is not known, it may be coated by any conventional method such as electroplating, electroless plating, electrophoretic deposition, vapor deposition, spraying, brushing or the like.

Using the techniques of this invention, the work piece can be coated by conventional coating procedures without any special provisions for avoiding the formation of the oxide of titanium. Moreover, the coating will be more adherently bound to the work piece than by prior art methods so that most coatings will not be readily peeled or chipped from the substrate and hence will not permit oxid-ative deterioration of the underlying titanium layer.

Among, but not limiting the materials which may be coated on the titanium or titanium alloy substrate whose surface is prepared as described herein are: ceramics, especially heat resistant ceramics such as the refractory oxides; paints such as zinc chromate paints, enamels, epoxies, primers and wash paints; adhesives such as glues, epoxy glues, rubbers and resins; and last but not least, metals such as nickel, cadmium, zinc, aluminum, chromium, platinum, gold, silver and iron, etc.

In a preferred embodiment of the invention where a metallic coating is to be applied on the titanium, a galvanic barrier is formed on the titanium surface. Thus, after pretreatment in the sulfuric acid chromic acid bath, the titanium substrate is treated with a nickel strike to deposit a layer of metallic nickel of the order of 0.001 inches thick. This is accomplished by fixing the part in a sulfamate nickel bath such as Barret, sulfamate nickel bath produced by Allied Research Corporation and Harshaw Chemicals, Harshaw sulfamate nickel. These are standard nickel plating baths containing roughly 9 ounces per gallon of nickel sulfamate. The nickel is generally deposited by electrolytic plating at from 25 to 50 amps per square foot of cathode area for a period of two minutes. This will deposit from 0.00005 to 0.0001 inches of nickel. After removal from the nickel plating bath, the piece is thoroughly rinsed and may then be plated with another metal, for example, cadmium from a standard plating bath by standard methods. Cadmium is plated in a preferred embodiment from a standard cadmium cyanide bath at 20 to 40 amps per square foot of cathode area to desired thickness. The purpose of this cadmium plating is to create a galvanic barrier. Typically, the desired thickness will be 0.0002 to 0.0001 inches thick. The nickel creates a barrier preventing stress migration of the cadmium. Metallic coatings deposited in this way are stable to standard bend, scratch or impact testing.

Having generally described the invention, a further understanding can be obtained by reference to the following specific examples which are intended for purposes of illustration only and are not intended to be limiting in any manner. All quantities are to be construed as parts by weight unless otherwise indicated.

Example l.A threaded fastener of 6-4 titanium is treated with Wyandotte F-S alkaline cleaner at a temperature of approximately 180 degrees Fahrenheit to remove organic soils until the surface is water break free. The work piece is then rinsed in deionized water, neutralized with a by volume sulfuric acid solution and again rinsed. The piece is then etched in a bath composed of 1.5 parts per gallon nitric acid and 3 fluid ounces per gallon hydrofluoric acid, for 10 seconds at room temperature. The remainder of the bath is water. The piece is removed, rinsed with deionized water and treated in a pretreatment bath of the following composition:

Chromic acid 36 ounces per gallon of water. Sulfuric acid 5 fluid ounces per gallon of water. Silver 0.6 troy ounces per gallon of water.

Aluminum sulfate 0.2 ounces per gallon of water.

The bath is preheated to about 240 degrees Fahrenheit and the work piece dipped via a titanium basket into the bath for 120 minutes. The work piece is removed from the bath, rinsed in cold deionized water and set aside.

Example 2.--Fasteners treated as in Example 1 are air dried and coated with enamel by spraying, brushing, or dipping. Good coatings are obtained which would not separate on bend testing or tape testing.

Example 3.--Piece treated as in Example 1 are brushed, sprayed or dip coated in a refractory oxide ceramic consisting of TiO and Cr O The coated piece is then fired. A wear resistant coating is obtained with no separation of the refractory on bend testing or on impact testing with an 8 pound weight dropped from 10 feet.

Example 4.-A titanium panel of dimensions 1 by 4 by 0.040 inch' is treated as in Example 1 through etching with a nitric hydrofluoric bath. The etched piece is rinsed and immersed in a pretreatment bath of the following composition:

Chromic acid 30 ounces per gallon. Sulfuric acid 7 fluid ounces per gallon.

The bath is preheated to 240 degrees Fahrenheit and immersion is for 45 minutes. The piece is then removed and rinsed.

Example 5.Fasteners treated as in Example 1 and a panel treated according to Example 4 are electroplated in a Barret sulfamate-nickel bath at 30 amps per square foot of cathode area for 2 minutes and then rinsed. This gives a nickel coating of approximately 0.00007 inches thick. These pieces are then electrolytically plated with cadmium from a standard cyanide cadmium plating bath at 25 amps per square foot of cathode area for 6 minutes to obtain a coating of 0.0002 to 0.0004 inches thick of cadmium. The coating is subjected to standard bend and drop testing procedures with no separation of cadmium. No stress migration is observed.

Example 6.A fastener of 64 titanium is treated with a nickel sulfamate strike as described in Example 5. The nickel plated substrate is then electroplated in a standard cadmium-fluoroborate plating bath at 15 amps per square foot of cathode area for 9 minutes. This gives a cadmium coating of 0.0002 to 0.0004 inches thick. No separation is observed on bend testing.

Example 7.A panel treated as in Example 5 is electrolytically plated with silver from a standard silver cyanide bath, at 10 amps per square foot of cathode area for 12 minutes. Deposition of 0.0003 to 0.0005 inch thickness of silver giving satisfactory results on bend testing is obtained.

Various changes and modifications of the embodiments of this invention may be made by those skilled in the arts without departing from the spirit and scope of the present invention.

Accordingly, what is claimed and intended to be covered by Letters Patent is:

1. A method of preparing titanium or titanium alloys in which titanium constitutes at least 50% of the metallic mix substrates for acceptance of a coating wherein said coating is adherently bound to said substrate, which comprises treating said titanium or titanium alloy substrate in a bath consisting essentially of 2 to 50 ounces per gallon of chromic acid and fluid ounce per gallon to 10 fluid ounces per gallon of sulfuric acid at a temperature above about 215 F. for a time sufficient to produce a black coating on the surface of said substrate.

2. The method of claim 1 wherein said bath contains from A to 2 ounces per gallon by weight of metallic silver.

3. The method of claim 1 wherein the bath contains approximately /2 gram per gallon of aluminum sulfate.

4. The method of claim 1 wherein said pretreatment bath comprises approximately 33 ounces per gallon of chromic acid and 4 fluid ounces per gallon of sulfuric acid.

5. The method of claim 4 wherein said bath additionally contains 1 ounce per gallon of silver and /2 gram per gallon of aluminum sulfate.

6. The method of claim 1 wherein said coating is selected from the group consisting of paints, adhesives, and ceramics.

7. The method of claim 1 wherein said coating is metallic.

8. The method of claim 7 wherein said metallic coating is selected from the group consisting of cadmium and chromium and is preceded by coating the pretreated substrate with a thin layer of nickel.

9. A method of preparing a titanium or titanium alloy in which titanium constitutes at least 50% of the metallic mix substrate for acceptance of a metallic coating which coating is adherently bound to said substrate, which comprises treating said titanium or titanium alloy substrate in a bath consisting essentially of 2 to 50 ounces per gallon of chromic acid and fluid ounce per gallon to 10 fluid ounces per gallon of sulfuric acid at a tempera- 7 8 ture above about 215 F. for a time sufiicient to produce 2,296,884 9/ 1942 Thompson 1486.21 a black coating on the surface of said substrate followed 2 433 510 0 9 9 Stareck X by treating said substrate in a nickel sulfamate strike, and then plating the desired metal thereon. 2524577 10/1950 Stareck 148562 10. The method of claim 9 wherein the metallic coat- 5 3,186,925 6/1965 Kushner 20441 ing is cadmium.

References m RALPH S. v KENDALL, Primary Examiner UNITED STATES PATENTS US Cl XR 2,946,728 7/1960 Foisel et al. 1486.2 X m 204 3g 3,210,220 10/1965 Clegg ..148-6.2X 

